Continuous process and plant design for conversion of biogas to liquid fuel

ABSTRACT

Biogases such as natural gas and other gases capable of being biologically derived by digestion of organic matter are converted to a clean-burning hydrocarbon liquid fuel in a continuous process wherein a biogas is fed to a reaction vessel where the biogas contacts a liquid petroleum fraction and a transition metal catalyst immersed in the liquid, vaporized product gas is drawn from a vapor space above the liquid level, condensed, and fed to a product vessel where condensate is separated from uncondensed gas and drawn off as the liquid product fuel as uncondensed gas is recycled to the reaction vessel.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of application no. 12/212,968, filed Sep.18, 2008, now U.S. Pat. No. 7,897,124, the contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1 . Field of the Invention

This invention lies in the field of biogas and its use as a source ofenergy, and also in the field of chemical plant design for conversion ofbiogas to liquid fuel.

2 . Description of the Prior Art

Alternative sources of energy are in ever increasing demand as crude oilfluctuates in price and as governments and the public at large becomeincreasingly concerned over adverse environmental impacts of gaseousemissions from the processing of crude oil. A major group ofalternatives to crude oil are the gases commonly known as “biogas” whichrefers generally to gases resulting from the decomposition of organicmatter in the absence of oxygen. The decomposition can occur in disposalfacilities for treating municipal waste and waste products in general,and the decomposition processes generally include anaerobic digestionand fermentation of biodegradable materials such as biomass, manure,sewage, municipal waste, and energy crops. The decomposition can alsooccur naturally in geological formations. Depending on its source,biogas can include hydrogen, methane, and carbon monoxide, as well asrelatively benign gases such as nitrogen and carbon dioxide. Natural gasis one form of biogas.

A co-pending United States patent application of potential relevance tothe present invention is Application No. 12/171,801, filed Jul. 11, 2008as a continuation-in-part of Application No. 12/098,513, filed Apr. 7,2008. The contents of both such applications as they relate toconversion of biogas to liquid hydrocarbon fuel are incorporated hereinby reference.

SUMMARY OF THE INVENTION

The present invention resides in a process scheme for converting biogasto a clean-burning liquid fuel (i.e., a liquid fuel that upon combustionproduces a gaseous combustion product that is at least substantiallyfree of particulate emissions and odor), and particularly a liquidhydrocarbon fuel, and to a processing plant designed to implement thescheme. The process scheme and plant operate in a continuous mode, andthe features of the scheme and plant include a gas-liquid reactionvessel and a product vessel, with a gas feed to the reaction vessel forinlet biogas and a port on the product vessel from which to draw liquidproduct. Fluid transfer conduits connect the two vessels, including onesuch conduit transferring vaporized product from the reaction vesselthrough a condenser and then to the product vessel, and another suchconduit transferring uncondensed gas from the product vessel back to thereaction vessel. Mounted inside the reaction vessel are a grid oftransition metal catalyst and gas distributors for both the feed gas andthe recycle gas, both under the liquid level. Optional features includea supplementary gas-phase reaction vessel downstream of the gas-liquidreaction vessel and upstream of the condenser, the supplementary vesselitself containing a grid of transition metal catalyst to react unreactedmaterials in the stream of vaporized product emerging from the reactionvessel. Further features of the plant are described below.

The reaction medium in the gas-liquid reaction vessel is a liquidpetroleum fraction, and the liquid product emerging from the productvessel is a hydrocarbon fuel of a composition that is distinct from theliquid petroleum fraction. The plant is operated on a continuous basis,and the reaction can be performed for a prolonged period of time,continuously producing product without adding further quantities ofliquid petroleum fraction to the reaction vessel, although such furtherquantities can be added as needed to supplement the liquid level orcompensate for liquid that has been entrained with the vaporizedproduct. In either case, the product is readily produced in a volumethat far exceeds the starting volume of the liquid petroleum fraction.

These and other objects, advantages, and features of the invention areincluded in the descriptions below.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a process flow diagram embodying an example of animplementation of the invention.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

The term “biogas” is used herein to include any non-inert gas that canbe produced by the biological degradation of organic matter. As notedabove, prime examples of biogas are hydrogen, methane, and carbonmonoxide, although other gaseous petroleum-based products such as ethaneand ethylene, and decomposition products of agricultural waste such aswood chips, grains, grasses, leaves, and the like, are likewise includedwithin the scope of the teem. The term is also used herein to includethe same gases that are obtained from other sources. One example ismethane associated with coal, commonly known as “coal bed methane,”“coal mine methane,” and “abandoned mine methane.” Such methane can bederived by bacterial activity or by heating. Gases containing 90% to100% methane on a mole percent basis are of particular interest, andthis includes natural gas, of which methane typically constitutesapproximately 95 mole percent.

The petroleum fraction in the liquid reaction medium in the process ofthis invention includes fossil fuels, crude oil fractions, and many ofthe components derived from these sources. The fossil fuels include anycarbonaceous liquids that are derived from petroleum, coal, or any othernaturally occurring material, as well as processed fuels such as gasoils and products of fluid catalytic cracking units, hydrocrackingunits, thermal cracking units, and cokers. Included among these fuelsare automotive fuels such as gasoline, diesel fuel, jet fuel, and rocketfuel, as well as petroleum residuum-based fuel oils including bunkerfuels and residual fuels. Fractions or products in the diesel range canalso be used, such as straight-run diesel fuel, feed-rack diesel fuel(diesel fuel that is commercially available to consumers at gasolinestations), light cycle oil, and blends of straight-run diesel and lightcycle oil. Crude oil fractions include any of the various refineryproducts produced from crude oil, either by atmospheric distillation orby vacuum distillation, as well as fractions that have been treated byhydrocracking, catalytic cracking, thermal cracking, or coking, andthose that have been desulfurized. Examples of such fractions are lightstraight-run naphtha, heavy straight-run naphtha, light steam-crackednaphtha, light thermally cracked naphtha, light catalytically crackednaphtha, heavy thermally cracked naphtha, reformed naphtha, alkylatenaphtha, kerosene, hydrotreated kerosene, gasoline and lightstraight-run gasoline, straight-run diesel, atmospheric gas oil, lightvacuum gas oil, heavy vacuum gas oil, residuum, vacuum residuum, lightcoker gasoline, coker distillate, FCC (fluid catalytic cracker) cycleoil, and FCC slurry oil. Preferred reaction media are mineral oil,diesel oil, naphtha, kerosene, gas oil, and gasoline.

The transition metal catalyst can be any single transition metal orcombination of transition metals, either as metal salts, pure metals, ormetal alloys, and can also be used in combination with metals other thantransition metals. Preferred catalysts for use in this invention aremetals and metal alloys. Transition metals having atomic numbers rangingfrom 23 to 79 are preferred, and those with atomic numbers ranging from24 to 74 are more preferred. Cobalt, nickel, tungsten, and iron,particularly in combination, are the most preferred. An example of anadditional metal that can be included is aluminum.

The metallic catalyst is used in solid form and is preferably maintainedbelow the liquid level in the reaction vessel as the biogas is bubbledthrough the liquid and through or past the catalyst. The catalyst canassume any form that allows intimate contact with both the liquidpetroleum fraction and the biogas and allows free flow of gas over andpast the catalyst. Examples of suitable forms of the catalyst arepellets, granules, wires, mesh screens, perforated plates, rods, andstrips. Granules and wires suspended across plates or between meshmatrices such as steel or iron wool are preferred for their relativelyaccessible high surface area. When granules are used, the granules canbe maintained in a fluidized state in the reaction medium or heldstationary in the form of a fixed bed. When wires are used, individualcobalt, nickel, aluminum, and tungsten wires, for example, ofapproximately equal diameter and length, and be strung across a frame ofcast iron to form an open-mesh network which can then be supportedinside the reactor. A reactor can contain a single frame strung withwires in this manner or two or more such frames, depending on the sizeof the reactor. A still further variation of the catalyst configurationthat can be used is a coil or other wrapping of the metallic wire aroundor over piping that serves as a gas distributor for incoming gas. Asmentioned above in the “SUMMARY OF THE INVENTION,” the reaction vesselwill typically contain one or more gas distributors for incoming gas,and in certain embodiments of the invention as explained below, thedistributor(s) may have a wheel-and-spokes configuration or any othershape that includes a network of hollow pipes with an array of aperturesto form the gas into small bubbles for release into the reaction vessel.These pipes, or at least the apertures, can be covered for example witha steel mesh or steel wool in combination with wires of the variousmetals listed above, to intercept the gas bubbles before they enter thereaction medium. The term “metallic grid” is used herein to denote anyfixed form of metallic catalyst that is submerged in the reaction mediumand allows gas to bubble through the grid. The term thus encompassesfixed (as opposed to fluidized) beds, screens, open-weave wire networks,and any other forms described above. The metal can be in bare form orsupported on inert supports as ceramic coatings or laminae.

The reaction is performed under non-boiling conditions to maintain theliquid petroleum fraction used as the reaction medium in a liquid stateand to avoid or at least minimize the amount of the liquid that isvaporized and leaves the reaction vessel with the product. An elevatedtemperature, i.e., one above ambient temperature, is used, preferablyone that is about 80° C. or above, more preferably one within the rangeof about 100° C. to about 250° C., and most preferably from about 100°C. to about 150° C. The operating pressure can vary as well, and can beeither atmospheric, below atmospheric, or above atmospheric. The processis readily and most conveniently performed at either atmosphericpressure or a pressure moderately above atmospheric. Preferred operatingpressures are those within the range of 1 atmosphere to 2 atmospheres.

The supplementary gas-phase reaction vessel referenced above as anoptional feature of the invention is a flow-through vessel with a gridof metallic catalyst, in which the term “grid” has the same scope ofmeaning as stated above in connection with the gas-liquid reactionvessel. In the supplementary vessel, however, the grid is not submergedin a liquid but instead supported within the vessel in the path of thevaporized product emerging from the gas-liquid reaction vessel. Themetals in the grid can be the same as those in the grid of thegas-liquid reaction vessel, or different combinations of transitionmetals.

A process flow diagram representing one example of a plant design forimplementation of the present invention is presented in the attachedFIGURE. The reaction vessel 11 and the product vessel 12 are both shown.Each of these vessels is a closed cylindrical tank with a volumetriccapacity of 2,000 gallons (U.S.) (7,570 cubic meters). The reactionvessel 11 is charged with a petroleum fraction used as a liquid reactionmedium 13 with a gaseous head space 14 above the liquid. The liquidlevel is maintained by a level control 15 which is actuated by a pair offloat valves inside the vessel. The level control 15 governs a motorvalve 16 on a drain line 17 at the base of the vessel.

Biogas is fed to the reaction vessel 11 underneath the liquid level atan inlet gas pressure of from about 5 psig to about 20 psig, through agas inlet line 18 which is divided among two gas distributors 21, 22inside the reactor vessel, each distributor being large enough todeliver 1,000 scfm of gas to the vessel. Each distributor spanssubstantially the full cross section of the vessel in either a gridconfiguration, a wheel-and-spokes configuration, or any otherconfiguration that will support an array of outlet ports distributedacross the cross section of the vessel. While two distributors areshown, the optimal number of distributors and outlet ports and theoptimal configuration for any individual distributor will be readilydeterminable by routine experimentation, with greater or lesser numbersof distributors being optimal for reactor vessels of differentcapacities. A resistance heater 23 is positioned in the reactor abovethe gas distributors, and a third gas distributor 24 is positioned abovethe resistance heater. The third gas distributor 24 receives return gasfrom the product receiving vessel 12 as explained below. The resistanceheater 23 maintains the liquid at a temperature of approximately240-250° F. (116-121° C.).

Positioned above the three gas distributors 21, 22, 24 and theresistance heater 23 but still beneath the liquid level are a series ofcatalyst grids 25 arranged in a stack. Each grid is a circular ring orapertured plate with metallic catalyst wires strung across the ring andsupported by pegs affixed to the ring along the ring periphery. Of thevariety of metals that can be used for the ring and the pegs, oneexample is a cast iron ring and chromium pegs. The sizes of the wiresand the total length of each wire will be selected to achieve themaximal surface area exposed to the reaction medium while allowing gasto bubble through, and will be readily apparent to anyone skilled in theuse of metallic or other solid-phase catalysts in a liquid-phase orgas-phase reaction. One example of a wire size is 1 mm in diameter.Using individual wires of each of four metals, such as for examplecobalt, nickel, aluminum, and tungsten, two pounds of each metal wirecan be used per ring, or eight pounds total per ring. The number ofrings can vary, and will in most cases be limited only by the size ofthe reactor, the gas flow rate into the reactor, the desirability ofmaintaining little or minimal pressure drop across the rings, andeconomic factors such as the cost of materials. In a preferredembodiment, seven rings are used, each wound with the same number andweight of wires. The reaction can also be enhanced by placing screens ofwire mesh between adjacent plates to assure that the gas bubblescontacting the catalyst wires are of a small size. Screens that are40-mesh (U.S. Sieve Series) of either stainless steel or aluminum willserve this purpose.

Product gas is drawn from the head space 14 of the reaction vessel 11and passed through a supplementary catalyst bed of the same catalystmaterial as the catalyst rings 25 of the reaction vessel. In the diagramshown, two such catalyst beds 31, 32 of identical construction andcatalyst composition are arranged in parallel. The supplementarycatalyst can be in the form of metallic wire screens, grids, orperforated plates similar to those of the catalyst grids 25 in thereactor vessel 11. The supplementary catalyst promotes the same reactionthat occurs in the reaction vessel 11 for any unreacted materials thathave been carried over with the product gas drawn from the reactionvessel. Product gas emerging from the supplementary catalyst beds ispassed through a condenser 33 and the resulting condensate 34 isdirected to the product vessel 12 where it is introduced under theliquid level.

The liquid level in the product vessel 12 is controlled by a levelcontrol 41 which is actuated by a pair of float valves inside the vesseland governs a motor valve 42 on a liquid product outlet line →at thebase of the vessel. Above the liquid level is a packed bed 44 ofconventional tower packings. Examples are Raschig rings, Pall rings, andIntalox saddles; other examples will be readily apparent to thosefamiliar with distillation towers and column packings. The packingmaterial is inert to the reactants and products of the system, or atleast substantially so, and serves to entrap liquid droplets that may bepresent in the gas phase and return the entrapped liquid back to thebulk liquid in the lower portion of the vessel. Unreacted gas 45 iswithdrawn from the head space 46 above the packed bed by a gas pump 47.The pump outlet is passed through a check valve 48 and then directed tothe reaction vessel 11 where it enters through the gas distributor 24positioned between the resistance heater 23 and the catalyst grids 25.

Alternatives to the units described above and shown in the FIGURE willbe readily apparent to the skilled chemical engineer. For example, anyknown type of condenser can be used to condense the vaporized productfrom the reaction vessel. Examples of types of condensers areshell-and-tube condensers and plate-and-frame condensers, and among theshell-and-tube condensers are horizontal tube condensers and verticaltube condensers. Either co-current or counter-current condensers can beused, and the condensers can be air-cooled, water-cooled, or cooled byorganic coolant media such as automotive anti-freeze (for example, 50%pre-diluted ethylene glycol) and other glycol-based coolants.Alternatives to the resistance heater are heating jackets, heating coilsusing steam or other heat-transfer fluids, and radiation heaters.Heating of the reaction vessel can also be achieved, either in part orin whole, by recirculation of heat transfer fluid between the coolantside of the condenser and the reaction vessel. The gas distributors forthe inlet feed and the recycle gas can be any of a variety of typesknown in the art. Examples are perforated plates, cap-type distributors,and pipe distributors. The liquid level controls can likewise be any ofa variety of mechanisms known in the art. Examples are float-actuateddevices, devices measuring hydrostatic head, electrically actuateddevices such as those differentiating liquid from gas by electricalconductivity or dielectric constant, thermally actuated devices such asthose differentiating by thermal conductivity, and sonic devices basedon sonic propagation characteristics.

EXAMPLE

This example illustrates the use of the present invention in aprocessing system in which the feed biogas was methane and the liquidpetroleum fraction used in the reaction vessel was diesel fuel. Theequipment was a pilot version of the plant set forth in the FIGURE anddescribed above, with a catalyst bed of aluminum wire, cobalt wire (analloy containing approximately 50% cobalt, 10% nickel, 20% chromium, 15%tungsten, 1.5% manganese, and 2.5% iron), nickel wire, tungsten wire,and cast iron granules. The reaction vessel was 19 inches (0.5 meter) indiameter and initially charged with ten gallons (39 liters) of dieselfuel. The diesel fuel was maintained at a temperature of 240-250° F.(116-121° C.) and a pressure of 3 psig (122 kPa) as the methane wasbubbled through the reactor. After startup, the reactor was run for tenhours, then continued for another 2.5 hours during which time productwas collected for analysis. The volume of product collected was 5.6liters, and upon completion of the collection, the volume of liquidreaction medium remained at 8-10 gallons (30-39 liters). The product wasanalyzed by standard ASTM protocols and the results are listed below.

Product Test Results Protocol Result Flash Point ASTM D 93 202° F. (94°C.) API Gravity at 60° F. (15.6° C.) ASTM D 287  34.8° PercentRecovered: Result Distillation at 760 mm Hg (1 atm) ASTM D 86 Initialb.p. 423° F. (217° C.)  5 452.5° F. (234° C.) 10 464.7° F. (240° C.) 20475.5° F. (246° C.) 30 485.4° F. (252° C.) 40 495.1° F. (257° C.) 50505.2° F. (263° C.) 60 516.0° F. (269° C.) 70 527.5° F. (275° C.) 80541.6° F. (283° C.) 90 560.8° F. (294° C.) 95 580.3° F. (305° C.) End597.9° F. (314° C.) Recovery 98.1%  Residue 1.0% Loss 0.9% Pressure 765mm Hg Estimated hydrogen content ASTM D 3343 13.38 weight % ParticulateMatter ASTM D 2276 2-0.8 μm filters 8.5 mg/gal Volume 0.26 gal Vacuum28.3 in. Hg Time 10 min Total Aromatics ASTM D 1319 18.0 volume %Sediment and Water ASTM D 2709 0 volume % Ash ASTM D 482 0.002 weight %Copper Corrosion ASTM D 130 1a (3 hours at 122° F., 50° C.) RamsbottomCarbon Residue, ASTM D 524 0.07 weight % 10% Bottoms Ramsbottom CarbonResidue ASTM D 524 0 weight % Lead ASTM D 3605 <0.1 ppm Vanadium ASTM D3605 <0.1 ppm Calcium ASTM D 3605 <0.1 ppm Sodium, Potassium, LithiumASTM D 3605 <0.1 ppm Demulsification ASTM D 1401 5 minutes Sulfur byX-ray ASTM D 2622 0.0005 weight % Cetane Number ASTM D 613 46.4 GrossHeat of Combustion ASTM D 240 19,547 BTU/lb, 138,490 BTU/gal

The product was fed to a VAL6 Infrared Oil Heater (Shizuoka Seiki Co.,Ltd., Japan) where it burned readily in air, emitting neither odor norsmoke.

In the claims appended hereto, the term “a” or “an” is intended to mean“one or more.” The term “comprise” and variations thereof such as“comprises” and “comprising,” when preceding the recitation of a step oran element, are intended to mean that the addition of further steps orelements is optional and not excluded. All patents, patent applications,and other published reference materials cited in this specification arehereby incorporated herein by reference in their entirety. Anydiscrepancy between any reference material cited herein and an explicitteaching of this specification is intended to be resolved in favor ofthe teaching in this specification. This includes any discrepancybetween an art-understood definition of a word or phrase and adefinition explicitly provided in this specification of the same word orphrase.

1. A continuous process for producing liquid fuel from a biogas containing 90 mole % to 100 mole % methane, said process comprising: (a) feeding said biogas to a reaction vessel partially filled with a liquid petroleum fraction to a first externally controlled liquid level to cause said biogas to bubble through said liquid petroleum fraction and through a metallic grid of at least one catalytic transition metal immersed in said liquid, while maintaining said body of liquid at a temperature of at least about 80° C. but below boiling; (b) passing a gaseous reaction product mixture from a head space in said reaction vessel above said liquid level through a condenser to convert said gaseous reaction product mixture to liquid condensate and uncondensed gas; (c) passing said liquid condensate and uncondensed gas into a product vessel having a second externally controlled liquid level, to separate said liquid condensate from said uncondensed gas; and (d) drawing from said product vessel said liquid condensate so separated as said liquid fuel while recycling said uncondensed gas from said product vessel to said reaction vessel by directing said uncondensed gas below said first liquid level and through said first metallic grid; steps (a) through (d) being performed continuously and simultaneously.
 2. The continuous process of claim 1 wherein said metallic grid in said reaction vessel is defined as a first metallic grid, said process further comprising passing said gaseous reaction product mixture through a second metallic grid of at least one catalytic transition metal prior to said condenser.
 3. The continuous process of claim 2 wherein said second metallic grid is external to said reaction vessel and said product vessel.
 4. The continuous process of claim 1 further comprising passing said uncondensed gas through a fixed bed of inert packing material to recover entrained liquid therein prior to recycling said uncondensed gas to said reaction vessel.
 5. The continuous process of claim 1 wherein said at least one catalytic transition metal constituting said first metallic grid is a plurality of transition metals with atomic numbers within the range of 24 to
 74. 6. The continuous process of claim 1 wherein said at least one catalytic transition metal constituting said second metallic grid is a plurality of transition metals with atomic numbers within the range of 24 to
 74. 7. The continuous process of claim 1 wherein said at least one catalytic transition metal constituting said first metallic grid and said at least one catalytic transition metal constituting said first metallic grid are both pluralities of metals comprising cobalt, nickel, and tungsten.
 8. The continuous process of claim 1 comprising maintaining said body of liquid at a temperature of from about 100° C. to about 250° C. in step (a).
 9. The continuous process of claim 1 comprising maintaining said body of liquid at a temperature of from about 100° C. to about 150° C. in step (a).
 10. The continuous process of claim 1 wherein said biogas is natural gas. 